The polymerization of ethylene in the presence of oxygen and/or free radical initiators in tubular reactors at high pressures and temperatures is well known in the art. It is also well known to increase the polymer productivity rate by introducing the initiator at more than one point along the length of the reactor, thereby establishing as many reaction zones within the tubular reactor as there are initiator injection points in the system.
Commercial size reactors are generally constructed of a plurality of tubular segments connected in series relation by blocks or other connection devices. The dimensions of the reactors include inside diameters broadly in the range of from about 0.5 to about 3 inches typically between about 1 and about 1.5, and total lengths of from about 800 to about 3000 feet or even longer. In order to confine the reactor system to an area of reasonable and practical dimensions, the tube is provided with many bends, e.g. in the fashion shown in FIG. 1 of U.S. Pat. No. 4,008,049, hereby incorporated into this specification by reference.
The polymerization reaction is highly exothermic and causes a rapid rise in the temperature along the length of a reaction zone until it reaches a peak when the initiator has been used up and polymerization discontinues. Cooling in one form or the other is required to control the reaction temperatures within safe and desired limits and to reduce the temperature of the reaction mixture to a suitable initiation temperature after which it is contacted with additional initiator in the subsequent reaction zone. It is therefore the usual practice to employ water-cooled jacketed reactors and in addition to introduce relatively cool side streams of the ethylene feed along the length of the reactor in cooling zones located between the reaction zones. In addition to the beneficial cooling effect achieved by the ethylene side stream introductions, further yield advantages are obtained thereby.
There are pressure fluctuations occurring in these tubular reactors employed for the production of polyethylene resulting in temperature changes within the reactor. Some of these pressure changes are incidental to reactions taking place during polymerization, but other pressure changes are purposefully employed to prevent accumulation of polymer on the interior walls of the reactor tube, these purposeful changes being known as "bump cycles" and being effected by the operation of "let-down" valves at the exit end of the reactor. This bump cycle may, for example, cause the reduction of pressure from 40,000 psi to 35,000 psi, this being a drop of 5,000 psi which causes shiftings of the temperature profiles throughout the reaction zones within the reactor tube, e.g. as depicted in FIG. 3 of the aforementioned U.S. Pat. No. 4,008,049.
Although in normal operations the reaction conditions in each reaction zone can be controlled rather precisely, a fortuitous upset in the initiator/monomer ratio or failure of control instruments can cause the temperature to rise within a reaction zone to levels where degradation of the product occurs and sometimes the degradation is so severe that the polyethylene product completely decomposes into carbon and hydrogen. The decomposition causes a dangerous and rapid increase in pressure to levels where the reactor tube might burst. It was generally believed that polyethylene decomposition occurred in the form of an explosion of considerable force involving pressure increases of up to about 500,000 psi per second. In order to minimize fire and explosion hazards to prevent serious damage to the equipment in case of a decomposition, as the pressure front proceeds within the reactor tube it has been the usual practice to install a multitude of rupture discs at regular intervals along the total length of the reactor tube. It was considered essential that the rupture discs were located immediately before the bends of the reactor tube to allow the pressure front to proceed in a straight line and be released through the rupturing disc without a change in direction. For this purpose the rupture discs were installed within connection blocks of a modified Y shape in a position perpendicular to the flow direction. Such blocks are shown in FIG. 1 of the aforementioned U.S. Pat. No. 4,008,049, e.g. by numerals 40 and 50, and the rupture discs were installed within the unconnected horizontal extension or arm of the Y block.
Although the aforementioned rupture discs function well in case of decomposition, the experience has been that in many instances a disc would rupture without any apparent reason for the failure, i.e. there were no indications of reactor condition upsets or degradation of the polymer product at the time of the rupture. In view of the considerable capital cost for each rupture disc installation and associated equipment, such as stacks and the production losses occurring during "down times" of a reactor it is therefore desirable to minimize both the number of rupture disc installations as well as the number of failures of the discs for causes other than polymer decomposition.
It is therefore an object of the present invention to provide a tubular high pressure polymerization reactor having a minimum of rupture discs without sacrificing safety. Another object of the invention is to provide a tubular high pressure reactor having rupture discs positioned in specific locations to minimize or obviate failures due to causes other than decomposition of polymer.
A further object of the invention is to provide a novel rupture disc-connection block assembly.